1. Field of the Invention
The present invention relates to a method of manufacturing a cylinder for a two-stroke internal combustion engine of relatively small size and to a cylinder for such a two-stroke engine. In particular, the present invention relates to a method for manufacturing an aluminum alloy cylinder which is provided with a pair of hollow scavenging passages and to a cylinder to be manufactured by the method.
2. The Prior Art
FIG. 6 illustrates one example of a cylinder for an air-cooled two-stroke gasoline engine of relatively small size that has been conventionally employed in a portable working machine. This conventional cylinder 1' is constituted by an integrated body having a cylinder bore 9 into which a piston (not shown) is adapted to be inserted, a block portion 2 provided with columnar bulged wall portions 2a, and a head portion 3 provided with a combustion chamber 4 of so-called squish dome-shape. Further, the block portion 2 is provided with a suction port 5 and an exhaust port 6, which face towards each other and are offset vertically from each other. The block portion 2 is further provided at the inner wall thereof with a pair of hollow scavenging passages 7 facing towards each other and displaced circumferentially from the suction port 5 and the exhaust port 6 by 90 degrees of arc. The scavenging passages 7 are formed in respective ones of the aforementioned columnar bulged wall portions 2a, thus forming hollow portions therein. A pair of openings 7a for introducing air-fuel mixture from a crank chamber (not shown) into the scavenging passages 7 are formed at the lower ends of the bulged wall portions 2a. Further, a pair of scavenging ports 8 are formed at the upper portions of the bulged wall portions 2a so as to communicate with the respective scavenging passages 7. Such a conventional cylinder 1' is of the so-called walled scavenging passage type cylinder structure.
In the mono-block casting of the cylinder 1', provided with the scavenging passages 7, by means of mold casting, either a collapsible core such as a shell core is employed to form the hollow scavenging passages 7 or a movable core mold which is complicated in structure is employed.
Since the scavenging passages 7 are provided with the scavenging ports 8, the mold is inevitably accompanied with an undercut portion for molding the scavenging ports 8. Thus in the mono-block casting of the cylinder 1' having scavenging passages 7, the employment of a core in addition to the mold is required in any event. As a result, there are problems in that the preparatory work for the casting is rather troublesome and the manufacturing cost for the cylinder is relatively high. In other words, it has been impossible to adopt an efficient casting method, such as a high-pressure die casting method, which is high in efficiency and low in manufacturing cost.
It may be conceivable to form the scavenging ports 8 by means of a contact working method using a revolving cutting tool, etc. However, since the space is too narrow to suitably insert the tool, the work is very difficult. Hence, it is difficult to expect a high precision working.